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MEMORANDUM Civil, Environmental Et Water Resources ENGINEERING CORP. TO: Kasey Foley, REHS Interim Director, San Joaquin Environmental Health Department FROM: Norman Hantzsche, PE, Questa Engineering Corporati DATE: December 20, 2019 SUBJECT: OWTS Design Review for 11878 West Clover Road, Tracy PROJECT OVERVIEW The project is for the permitting of a 2 -bedroom second unit dwelling on a 0.91 acre lot having a 3 -bedroom main residence. Plans and supporting documentation for the OWTS are contained in the "Nitrate Loading Study, by Chesney Consulting, dated July 13, 2017, and a follow-up OWTS design plan for the second unit dwelling dated August 23, 2019 (also by Chesney Consulting). The Nitrate Loading Study estimated the resultant nitrate -nitrogen loading to be 23.3 mg-N/L, in excess of the 10 mg-N/L criterion. County Environmental Health reviewed and accepted the Nitrate Loading Study and required that an Altemative/Engineered OWTS be provided to reduce the nitrate -nitrogen discharge to an acceptable level. The consultant's August 23, 2019 report proposed the placement of highly organic peat or wood chip type material along the bottom (floor) of the leachfield filter bed to mitigate the high nitrogen loading. The County requested my review of the proposed nitrogen mitigation design. COMMENTS Nitrogen Loading Study. In evaluating the nitrogen mitigation design I first reviewed the 2017 Nitrate Loading Study, which had been accepted by the County. I found the projected resultant nitrate -nitrogen value of 23.3 mg -NIL to be overly conservative, due mainly to the use of a very high value for estimated wastewater flow, 700 gpd for the two residences combined. This may be an appropriate value for peak flow considerations, but is far more than would be expected on an average basis, which should be used for the nitrate loading analysis. Average flows of 150 gpd per primary and secondary dwelling (300 gpd total) would be more appropriate assumptions for this analysis. Substituting the lower value for wastewater flow along with the other assumptions in the Chesney report, gives a lower estimate for the resultant nitrate -nitrogen loading of 14.4 mg-N/L. This result is below the estimate by Chesney (23.2 mg-N/L) but still significantly above the 10 mg-N/L criterion and confirms the need for nitrogen mitigation. 2. Proposed Nitrogen Mitigation. The proposed use of organic material to line the bottom of the leaching filter bed is a highly questionable approach to nitrogen removal and not Box 70356, 1220 Brickyard Cove Rd. Suite 206 Pt. Richmond, CA 94807 T.., 510/236.6114 F: 510/236.2423 E: Questa®QuestaEC.com